Effect of Solution Temperature on Microstructure and Properties of 022Cr22Ni5Mo3N Duplex Stainless Steel

Effect of Solution Temperature on Microstructure and Properties of 022Cr22Ni5Mo3N Duplex Stainless Steel

After solution at 1060 ℃, 2205 steel has a tensile breaking strength of 731MPa, elongation of 38%, and impact energy of A_kv (- 40 ℃) of 76-81J, meeting the standard requirements.

With the increasing demand for energy, the exploration and development of oil and gas wells are facing more technical problems. Oil well pipe long-term service in corrosive media and alternating load conditions, for this reason, Sweden developed for acid oil well pipe and pipeline material SAF2205 steel, China in the early 1980s developed and SAF2205 steel chemical composition close to 022Cr22Ni5Mo3N duplex stainless steel. 022Cr22Ni5Mo3N steel is an austenitic-ferritic duplex Stainless steel, the steel by the ferrite and austenite two phases in a certain ratio, two-phase coexistence structure, both ferrite and austenitic stainless steel advantages, the alloy composition and microstructure characteristics of the steel has a high mechanical tensile strength, stress corrosion resistance, low temperature impact toughness, the steel is widely used in the production of petroleum, chemical, paper and energy industry and other fields. In this paper, the microstructure and mechanical properties of 022Cr22Ni5Mo3N steel were studied at different solution temperatures, and the effect of heat treatment solution temperature on the microstructure and properties of the material was analyzed.

1. Test method and procedure

The main chemical composition of the ingot is shown in Table 1, and the composition is in accordance with the relevant standards of ASTMA240UNSS32205-2007. In order to control the appearance of excess ferrite caused by high forging heating temperature and long holding time, the billet hot forming heating interval and billet holding time must be strictly controlled. The ingots were heated and held for 4-6h at 1150-1170°C and then forged with a 750kg forging hammer. The final forging temperature was about 970°C. The test billets were air cooled after forging. Then the test billet was cut by sawing, firstly, the metallographic specimens were cut with the size (mm) of 15×15×15 to observe the original tissue state of the billet after forging, and then different heat treatment processes were adopted for different test billets as shown in Table 2.
After the test billets were treated with different heat treatment temperatures, the test specimens were cut from different test billets A and B on the sawing machine, respectively, and the metallographic specimens were (mm) 15 × 15 × 15, tensile specimens and impact specimens were taken at 1/2 radius from the heat treatment surface of the test billets, and the direction was longitudinal. Tensile specimens were processed with a diameter of 10 mm and a standard tensile specimen with a standard distance of 50 mm, and impact specimens were made with a Charpy V-notch. The mechanical properties and organization of the billet specimens obtained by solid solution at different heat treatment temperatures were discussed.
Table.1 Main chemical composition of test steel 022Cr22Ni5Mo3N/%

C	Si	Mn	P	S	Cr	Ni	Mo	N
0.021	0.38	0.59	0.012	0.008	21.31	5.32	3.02	0.13

Table.2 Different heat treatment temperatures of the test billets

Test No	Solution process
A	1 000 ℃ 4h, water-cooled
B	1060 ℃ 4h, water-cooled

2. Test results and their analysis

Figure l (a, b, c) are the original state of the test billet after forging and the metallographic microstructure of billet A, B after solid solution heat treatment. In Figure 1, the white part is the austenite phase, and the gray part is the ferrite phase. It can be seen that the organization of the test billet after forging is composed of α ferrite + austenite + grain boundaries and intergranular precipitates. At this time, the original organization of the billet after forging, α ferrite phase accounts for about 31% of the volume fraction, austenite phase accounts for about 56% of the volume fraction, and precipitates constitute about 13% of the volume fraction. After solid solution treatment of test blanks A and B, their microstructures were composed of ferrite + austenite two-phase organization, in which the α-ferrite phase accounted for about 38% of the volume fraction, the austenite phase accounted for about 60% of the volume fraction, and the precipitate composition accounted for about 2% of the volume fraction; after solid solution of billet B, the α-ferrite phase accounted for about 50% of the volume fraction, and the austenite phase accounted for about 50% of the volume fraction. volume fraction.

Figure.1 Histomorphology of 022Cr22Ni5Mo3N steel Φ75mm (a) forging, (b) solid solution at 1000℃ for 4h, water cooling and (c) solid solution at 1060℃ for 4h, water cooling, ×100
Table.3 Comparison of mechanical properties of 022Cr22Ni5Mo3N steel specimens after solid solution heat treatment at different solid solution temperatures

Project	Rp0.2/MPa	Rm/MPa	A/%	Z/%	AKv(-40℃)/J
Standard	–	≥700	≥25	–	≥50
1000 ℃ solid solution (A)	583	729	21	51	28,31 ,29
1060 ℃ solid solution (B)	567	731	38	71	79,76,81

Table 3 shows the mechanical properties of 022Cr22Ni5Mo3N steel specimens after solid solution at different heat treatment temperatures, and the comparison of the mechanical properties of specimens A and B shows that there are some differences in the measured mechanical properties, especially in the elongation, section shrinkage, yield strength and low temperature impact toughness of the material.
Comparing the metallographic organization Fig. 1(b,c), it can be seen that the austenite phase content in the microstructure of test billet A is significantly more than that of billet B. In addition, it is observed that the austenite phase content in the microstructure of test billet A is significantly more than that of billet B. In addition, some black precipitates were observed inside the grains and along the grain boundaries of ferrite and austenite phases in the microstructure of test billet A. The precipitates were mainly distributed along the grain boundaries in the form of continuous strips and a small amount of intracrystalline distribution, while no precipitates were observed inside the grains and along the grain boundaries of ferrite and austenite phases in the microstructure of test billet B. In addition, no precipitates were observed inside the grains and along the grain boundaries of ferrite and austenite phases in the microstructure of test billet A. In addition, the distribution of ferrite and austenite in the metallographic organization of specimen A is relatively disordered, in which the ferrite phase is dominated by small-angle grain boundaries and the austenite phase contains part of the twin grain boundaries, i.e., the ferrite and austenite phases are dominated by low-energy special grain boundaries, while the austenite and ferrite phases in the metallographic organization of specimen B are distributed in strips. The results showed that the precipitates were rich in Cr, Mo elements, the composition of (FeNi)_x(CrMo)_y, that is, the σ phase, the phase mainly along the ferrite and austenite two-phase grain boundary and part of the ferrite crystal precipitation, the precipitation of the σ phase, so that the concentration of impurities on the grain boundary Further increase, the grain boundary is the material microscopic defect gathering area, the increase in impurity concentration to the grain boundary is weakened. When the material is subjected to stress load, the stress causes the internal dislocation slip movement, dislocation movement to the σ-joint plugging, so that the formation of a large stress concentration around the grain boundary, when the stress concentration at the grain boundary, the grain boundary resistance to crack expansion is further weakened, these large amounts of σ-phase precipitation along the grain boundary reduces the ability of the grain boundary to withstand the load, increasing the stress sensitivity of the grain boundary, resulting in the low-temperature impact toughness of the material The notch sensitivity increases, making the measured impact work of specimen A lower. In addition, when the tensile test of specimen A is subjected to tensile stress, the material will selectively dislocation slip at the weak microstructure, especially at the σ-phase grain boundaries, forming stress concentrations, microcracks and crack expansion. The values show a certain variation pattern.
Table.4 Results of EDS analysis of 022Cr22Ni5Mo3N steel after solid solution at 1000℃ for 4h/%

Project	Fe	Si	Mn	Cr	Ni	Mo
Austenite	67.13	0.31	1.59	20.31	7.28	1.87
Ferrite	67.57	0.68	1.29	23.14	2.73	2.89
Precipitated phase	56.47	0.89	1.31	28.91.	2.76	8.83

The microstructure of test billet A has more small-angle grain boundaries, which contain more twins and dislocations that prevent each other from moving, and the dislocation climbing process is constantly plugging, and the metal atoms such as Cr and Mo are biased on the dislocation line, forming a lattice distortion and dislocation strengthening effect, so the yield strength measured in test billet A is higher than that of test billet B. With the increase of solid solution temperature, the activity of each metal atom increases, With the increase of solid solution temperature, the activity of metal atoms increases, and the diffusion rate of Cr and Mo on the dislocation line is accelerated under the temperature effect, and the dislocation expansion is formed continuously, Cr and Mo are dissolved in the solid solution matrix, and the σ phase is dissolved in the solid solution continuously, and the dislocation density decreases, and the twin crystal and small angle grain boundary is reduced. Therefore, the elongation and impact toughness of specimen B are much higher than those of specimen A. The above test results show that the overall performance of specimen B is significantly better than that of specimen A.

3. Conclusion

022Cr22Ni5Mo3N steel after solid solution water cooling at 1000 ℃, the microstructure of ferrite and austenite two-phase grain boundaries and intracrystalline precipitation of σ harmful brittle phase, the phase in the grain boundaries is semi-continuous distribution, affecting the material after solution heat treatment of elongation, low temperature toughness and other mechanical properties of the material, the steel after solid solution water cooling at 1060 ℃, the microstructure of uniform, intracrystalline and ferrite and austenite two-phase grain boundaries. The elongation and low-temperature toughness of the material were improved to different degrees, and the tensile fracture strength, elongation and impact work of the material all reached the technical requirements, which improved the tensile performance of the material.

Source: China 2205 Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)